In satellite digital radio systems audio signals are digitized and transmitted from satellites using digital communication techniques, such as digital modulation and coding. A satellite digital radio system offers benefits that are not available with conventional AM or FM analog radio systems. Digital compression techniques allow satellite digital radio systems to provide high-quality audio signals, even when the signals are received in a moving vehicle. High-quality audio reproduction is generally not possible with conventional AM or FM analog radio systems.
Particularly troublesome for satellite digital radio reception is signal degradation due to multipath fading, which is essentially a variation in RF signal levels due to multiple random signal reflections. Although baseband digital transmission techniques, such as equalization and modulation, can lower the impact of multipath fading, degradations still exist.
In particular, serious signal degradations may occur when a satellite digital radio receiver is indoors or in narrow inter-building alleys. The digital signals from orbiting satellites may not be acquired with a good line-of-sight path. Consequently, numerous random reflections of the satellite-originated signals, i.e., the multipaths, may be the only signals that the satellite digital radio receiver radio can receive.
To reduce the impact on radio reception by degradations such as multipath fading and other types of signal blockage, several transmission redundancy techniques, collectively called diversity techniques, are used in current satellite digital radio systems. A first technique is satellite spatial diversity in which two or more satellites transmit identical signals from widely spaced apart locations. A second technique is frequency diversity in which different satellites transmit the same signals at different frequency bands. A third technique is time diversity in which different satellites transmit the same signals at slightly different times. In hard-to-reach areas, such as dense urban centers or in signal-blocking structures such as tunnels, the satellite digital radio signals are re-transmitted at separate frequencies using terrestrial repeaters.
Current subscriber device antenna systems use omni-directional antennas that are adequate when relatively strong line-of-sight signals are available from either the satellites or from the terrestrial repeaters. However, omni-directional antennas perform very poorly in multipath-rich environments, such as inside buildings or in narrow inter-building locations in dense urban areas. A single fixed beam antenna has no way of determining the direction from which a reflected radio signal can be best received, and cannot be pointed to more accurately detect and receive a signal in any particular direction.
Current antenna systems also use dual element antennas commonly referred to as diversity antennas. Although performance can be improved in certain situations, the dual element antennas can also be susceptible to multipath fading due to the symmetrical nature of the hemispherical lobes formed by the antenna pattern. A signal reflected in a reverse direction from its origin can be received with nearly as much power as the original signal that is directly received. That is, if the original signal reflects from an object beyond or behind the intended receiver (with respect to the sender) and reflects back at the intended receiver from the opposite direction as the directly received signal, a phase difference in the two signals can create a multipath fading situation.
It is also common to have fixed directional high gain antennas pointing toward the satellite, i.e., typical outdoor antennas. Since these antennas have a fixed pointing angle, they can not adapt well to multipath environments where the best signals may be a reflected signal which is not in the direction of the direct satellite path.
Another problem is the reception of signals that have been retransmitted by terrestrial repeaters, in which these signals may be interfered with by nearby transmitting systems. Although the signals may be separated in the frequency spectrum, adjacent channels may still experience interference from nearby transmitter stations. Consequently, omni-directional antennas and dual element diversity antennas are not adequate for receiving signals in multipath environments.